The present invention relates to a power train, and more particularly to a power train for driving the road wheels and the marine propulsion means of an amphibious vehicle. The invention also relates to an amphibious vehicle having such a power train.
In an amphibious vehicle it is advantageous to use a power train in which an engine and transmission are positioned towards the rear of the vehicle. The weight of the power train is therefore positioned towards the back of the vehicle, which is necessary for good vehicle performance when the vehicle is in marine mode. Furthermore, the position of the power train maximises the space available towards the front of the vehicle for the passenger compartment.
A power train Is disclosed for use in an amphibious military personnel carrier in U.S. Pat. No. 5,752,862 (Mohler). The disclosed power train uses a rear mounted engine and a combined transmission and differential mounted at the front of the vehicle. Although Mohler uses the drive shafts from the differential to drive caterpillar tracks, such drive shafts could also be used to provide drive to the front road wheels of a civilian amphibious vehicle. However, the power train arrangement disclosed in Mohler has several disadvantages for application to such a civilian vehicle. First, the location of the transmission at the front of the vehicle may reduce passenger and/or luggage space. Secondly, the arrangement requires a propeller shaft running through the passenger area. This shaft, which carries full engine power and rotates at engine speed, takes up valuable passenger space and will generate noise. Third, front, wheel drive is not an optimal solution for a rear engined vehicle, as traction will not be as good as if the engine weight is placed over the driven wheels. This can give problems for example intake-off on slippery surfaces, and unusual on-road handling characteristics. There are some circumstances where driving front wheels only may be advantageous, e.g. when exiting water; but it is preferred that this is not the only option available to the driver.
Other power train arrangements for use in an amphibious vehicle are known from U.S. Pat. No. 5,590,617 (Aquastrada) and U.S. Pat. No. 3,765,368 (Asbeck). In these power trains an engine and transmission are connected end-to-end in conventional automotive rear wheel drive fashion, but with the overall arrangement reversed to drive the front wheels. As can be seen particularly clearly from Aquastrada, this forces the passenger seating area towards the front of the vehicle, followed by a long rear deck area which cannot be used for passenger or luggage space.
It is an object of the invention to resolve problems in packaging and traction in the prior art solutions, reducing the lengthwise space taken up by the power train.
According to a first aspect of the present invention, there is provided a power train for an amphibious vehicle comprising an engine and a transmission, the engine and the transmission being positioned with the axis of the crankshaft of the engine offset from and substantially parallel to the axis of an input drive shaft of the transmission, the arrangement being such that the power train can be positioned in the vehicle with both of said axes substantially parallel with the longitudinal axis of the vehicle; characterised in that the crankshaft of the engine longitudinally overlaps at least part of the input drive shaft of the transmission.
The invention provides a power train for an amphibious vehicle whose overall length is reduced when compared with the prior art amphibious vehicle power trains. This allows the vehicle designer to maximise the available passenger area for a vehicle of any given length. Furthermore, the inventive arrangement is particularly suited for mounting the engine and transmission towards the rear of the vehicle giving a rearward weight bias suitable for good marine performance with optimized traction at the driven rear wheels.
Preferably, the engine and transmission are positioned so that the axis of the engine crankshaft is offset above, to one side, or above and to one side of the axis of the input shaft of the transmission.
Conveniently, the input drive shaft of the transmission also drives a drive shaft of a marine propulsion means. A decoupler may be provided to selectively couple and decouple the drive from the input drive shaft of the transmission to the drive shaft of the marine propulsion means. Preferably, the marine propulsion means is a water jet unit. Alternatively, the marine propulsion unit may comprise a marine screw propeller.
Preferably the engine and transmission are adapted for mounting towards the rear of the vehicle and to provide drive to at least the rear wheels of the vehicle. The transmission may be adapted to provide drive to all the road wheels of the vehicle. The arrangement may be such that drive to the front wheels can be selectively disconnected whilst maintaining drive to the rear wheels only and vice versa.
Drive can conveniently be transmitted between the crankshaft and the input drive shaft of the transmission by means of a belt or chain. In a preferred embodiment, a first sprocket is mounted for rotation with the crankshaft and a second sprocket is mounted for rotation with the transmission input shaft, with drive being transmitted between the two sprockets by means of the belt or chain. Alternatively, gears may be used to transmit drive from the crankshaft to the transmission input shaft.
According to a second aspect of the invention, there is provided an amphibious vehicle having a power train in accordance with the first aspect of the invention.